The invention relates to point-to-multipoint transmissions in a mobile communication system and, more particularly, to point-to-multipoint transmissions which have to be acknowledged.
Mobile communication systems have been developed in order to free people from fixed telephone terminals without hindering their reachability. Coinciding with the increased use of various data transmission services at offices, a plurality of data services has also appeared in mobile communication systems. Mobile networks for their part provide a user with an efficient access network for mobile data transmission, the network giving the user access to the actual data networks. On this account, various new forms of data services are being designed for the current and future mobile communication networks. Digital mobile communication systems, like the Global System for Mobile Communication GSM, are particularly suitable for supporting mobile data transmission.
The General Packet Radio Service GPRS is a new service in the GSM system and one of the objects of GSM Phase 2+ standardization at ETSI (European Telecommunication Standard Institute). The GPRS allows packet data transmission to be established between mobile data terminals and external data networks, with the GSM network functioning as an access network. One of the requirements set for the GPRS is that it must interwork with different types of external data networks, such as the Internet or the X.25. networks. In other words, the GPRS and the GSM network should be able to serve all users, irrespective of which type of data networks the users wish to enter through the GSM network. This means that the GSM network and the GPRS must support and process various types of network addressing and data packet formats. The processing of data packets also comprises their routing in a packet radio network. Further, users should be able to roam from the GPRS home network to another GPRS network, whose operator backbone network may support a protocol (e.g. CLNP) different from that of the home network (e.g. X.25). The GPRS network architecture is illustrated in FIG. 1.
The GPRS supports both point-to-point and point-to-multipoint transmissions. The aim of a point-to-multipoint transmission is to allow a sender to transmit data to recipients in a destination area by using one service request. The term ‘data’ refers in this application to any information to be conveyed in a digital telecommunication system. The information may comprise digitized speech, inter-computer data communication, telefax data, short program code segments etc. The destination area is a geographical area determined by the sender. The destination area is determined either in the service request or when the starting of a point-to-multipoint transmission is notified.
To control point-to-multipoint transmissions the GPRS network typically comprises a Point-To-Multipoint Service Centre PTM-SC, which is an essential element in the point-to-multipoint service. The centre receives service requests from a Service Requester and transmits the service to its service area via the Serving GPRS Support Node SGSN. Actual point-to-multipoint services supported by the GPRS are Point-to-Multipoint Multicast PTM-M and Point-To-Multipoint Group call PTM-G. In the GPRS system, the term ‘group’ refers to several mobile stations which have registered with the same International Mobile Group Identifier IMGI. Groups can either be open or closed. An open group can be joined by anyone, whereas a closed group includes only the subscribers who have been defined to belong to the group in the service centre PTM-SC. Besides the actual point-to-multipoint transmissions, the GPRS supports IP Multicast IP-M according to the Internet protocol.
A PTM multicast PTM-M is broadcast in all the cells belonging to the destination area. It can be directed to all mobile stations in the cells or to mobile stations belonging to a certain group. A PTM multicast is unidirectional, non-encrypted and unreliable. Thus, anyone can listen to the transmission and the sender cannot know, whether the receiver/s has/have received the message. A message to be transmitted as a PTM multicast includes scheduling information. Scheduling information comprises the starting time, the end time and the frequency rate of the transmission. If the starting time zero is given, it deals with a real time transmission. Real time means that a message received from the service requester is transferred as quickly as possible. Transmission rate and transmission time delay vary depending on the loading of the network elements. If each piece of scheduling information is marked with zero, it deals with a real time single transmission. If the starting time is other than zero, it deals with a delayed transmission. On the basis of the time difference between the starting and end time and of the frequency rate, the service centre PTM-SC calculates the number of transmission repetitions and the time slot between the repetitions. By using this information, PTM-SC controls the transmissions of the message. The end time is only used in calculating the control information of the above mentioned transmissions.
A PTM group call PTM-G is transmitted in the cells of the destination area which include at least one mobile station registered to the group. Only a mobile station registered to the group in the area of a serving support node SGSN can receive messages of a group call and decode the encryption. Thus, the network is aware of the location of the registered mobile stations. A PTM group call can be transmitted as a broadcast, a point-to-point transmission or as a combination of these. A group call is always individualized by the mobile group identity IMGI. In a PTM group call, a transmission is either uni-, bi- or multidirectional, encrypted and reliable. Usually the messages of a PTM group call are transferred in real time. It is also possible to employ a delayed transmission and/or repeated transmissions as in the PTM multicast. Since a PTM group call is reliable, at least a broadcast group call must be acknowledged. In case of a negative acknowledgement, a mobile station transmits the acknowledgement only if it notices that it has not received the previous PTM message or messages. In such a case, the service centre transmits the missing messages to it. In case of a positive acknowledgement, each PTM message is acknowledged individually. A positive acknowledgement is especially applicable to cases in which the reliability requirements are strict. In both manners of acknowledging, each acknowledgement transmitted by a mobile station is conveyed via the serving support node SGSN to the service centre, which decides on the following actions on the basis of the acknowledgements. At the end of the PTM group call, the service centre PTM-SC transmits a report to the service requester.
On the basis of what is described above, a problem arises that a point-to-multipoint message can only be transmitted after the content of the message has already gone out of date. This is the case particularly in the transmissions which have to be repeated and transmitted as scheduled. On the other hand, a group message which has to be acknowledged cannot be delivered to the mobile stations which have not received it at the time of the actual transmission, although the mobile stations arrived at the destination area during the time the content of the message has not yet gone out of date.